Conventionally, devices in which a plurality of LED elements have been disposed in a linear or planar shape have been used in photograph processing devices (film scanners) and the like. Patent Document 1 discloses a well-known example of such a device. Patent Document 1 discloses a device which divides a plurality of LED elements into groups of at least two or more LED elements that are interconnected, and which is provided with a forward voltage drop measuring method that measures the total forward voltage drop of the respective LED elements that form the respective groups. The forward voltage drop measuring method includes: a voltage detection unit for each of the respective groups that extracts a total forward voltage drop of the respective LED elements forming the group; a method for measuring the forward voltage drop extracted by the voltage detection unit; a method for comparing the measured forward voltage drop to a reference voltage; a method for determining whether or not the forward voltage drop satisfies a prescribed judging standard in accordance with the comparison result; and a method for displaying the determination result.